During papermaking processes there is a desire to obtain strong paper and board products but yet with a high bulk (low density). The advantage with high bulk products is that these products require smaller amounts of fibers, which is economically favorable. Furthermore, high bulk board products with low fiber content have better bending stiffness which is an advantage during creasing and forming of packages from the board product.
In order to increase the bulk of a product, pulp comprising long fibers and as little fine material as possible should be used. However, pulps that are rich in stiff, long fibers unfortunately show poor ability to produce sufficient strong paper or board products. The bulk and strength properties of the resulting paper or board will therefore be a compromise between the ability of the pulp to increase the bulk and its ability to increase the fiber bonding properties, i.e. strength, of the paper or board.
A common way to produce a high bulk product is to use of chemi-mechanical pulp (CTMP) in the product, preferably in the midply of a multiply paper or board product. Also, mechanical fibers can be used in order to increase the bulk of a product.
One way to increase the fiber bonding properties of mechanical pulps is by addition of chemical additives to the pulp. The predominant treatment for improving the strength, particularly dry strength, of paper or board has so far been to add cationic starch to the pulp fiber slurry prior to the sheet forming operation. It is however difficult to adsorb large amounts of starch to the fibers, especially when the fines amount is small.
Another way to increase the bulk in a multiply paper or board product is described in EP0511186A1. A slurry comprising cellulosic fibers that has been heat treated is used for the production of a product with reduced density (i.e. decreased bulk) but still with good stiffness and bending ability. The slurry is treated at an elevated temperature of 200-240° C., for a period of 0, 1-10 minutes and at a dry content of 3-4%. However, this is a very energy consuming process since it takes a lot of energy to heat the water present in the slurry.
Thus, there is still a need for an improved process for production of a paper or board product with increased bulk but still with good strength properties.